DESCRIPTION: The mechanism behind allograft (Tx) acceptance may include a process termed "infectious tolerance"(IT). IT propagates itself into new sets of recipients by transferred mononuclear cells (MNC). Lew rats sensitized with BN skin Tx reject donor-specific cardiac Tx in <36 hr, yet maintain them for >200 days after treatment with CD4 mAb (RIB-5/2). This effect is accompanied by features characteristic for the IT pathway. The PI will use this rat Tx model to define key mechanisms contributing to IT. He hypothesizes that acquisition of IT to MHC-incompatible organ Tx results from host mechanisms controlled by regulatory CD4+ T cells. Once established, the Th2-type response becomes dominant and instrumental to the maintenance of well-functioning Tx in tolerant recipients. The PI plans to address the following questions: (1) What are the identity and functional characteristics of regulatory T cells mediating IT? He will fractionate MNC from RIB-5/2 mAb-treated cardiac Tx recipients into phenotypically distinct subsets and evaluate their efficacy to confer tolerance into new cohorts of test cardiac Tx recipients. These tolerance-conferring cells will be also studied for their ability to overcome allorecognition properties in "mixing" experiments, following re-transplant of "parked hearts" into new sets of test recipients, and in thymectomized hosts. The recirculatory routes of transferred cells will be identified. He will also analyze the functional profile of tolerant cells in vitro (MLR, CTL, cytokine elaboration). (2) What is the role of intra-Tx cytokine network in IT? First, the PI will test whether infusion of anti-IL-4 mAb of rIFN-g/rIL-2 may prevent tolerance induction, and recreate cardiac Tx rejection in test recipients. He also proposes to use in vivo gene transfers utilizing IL-4 and IFN-g encoding replication-deficient adenoviral vectors. He will determine whether intra-Tx overexpression of IL-4 may boost the Th2-type milieu, and amplify the IT pathway. He will assess how cytokine gene transfer affects intra-Tx cellular infiltrate/activation and Th1/Th2 cytokine expression profile. This approach will be then employed in an attempt to counteract the IL-4-mediated effects by intra-Tx overexpression of the IFN-g gene. (3) How does local expression of cytokine-controlled allo-Abs and "protective" molecules affect Tx injury in tolerant hosts? Th2-mediated depression of certain IgG isotypes in parallel with upregulation of protective molecules (Bcl-2, Ccl-xL, A-20, HO) may contribute to the avoidance of chronic Tx rejection in tolerant hosts. The PI will test whether manipulations within the Th1/Th2 cytokine network, as proposed in Aim 2, will modify intra-Tx deposition of IgG isotypes and expression of protective proteins, with resultant avoidance of Tx arteriosclerosis and chronic rejection in the IT pathway.